The present invention relates to protective relay apparatus for electrical power systems and methods of such protection. More particularly, the present invention relates to overexcitation relay apparatus and methods of overexcitation protection.
Overexcitation is excessive magnetic flux density which saturates the magnetic cores of protected equipment such as generators, transformers, and iron core reactors. When a magnetic core is saturated by an alternating current (AC) source, any increase in flux density greatly increases the amount of heat generated in the core. Modern equipment designs are especially sensitive to overexcitation because they normally operate with high flux densities. Automation of substations and generating facilities is also increasing the need for overexcitation relaying.
The magnetic cores of power system equipment typically have silicon steel laminations to reduce eddy currents. However, during overexcitation the eddy currents in the core become a significant factor in the heating of the equipment. Leakage or stray flux also enters nonlaminated parts such as structural steel of the generators, transformers, and reactors to produce substantial eddy current losses there also. Overheating causes severe damage and equipment failure by deteriorating electrical insulation in the equipment.
The voltage across a winding on the magnetic core of protected equipment is, according to a basic physical principle known as Lenz's Law, proportional to the time-derivative of the flux density. Consequently, the flux density is proportional to the time-integral of the voltage across the winding. In an AC electric power system in which the voltage is essentially sinusoidal, the time integral of the voltage is, by elementary calculus, proportional to the ratio of the voltage to the frequency (in Hertz). Consequently, an overexcitation relay is also called a volts-per-Hertz (V/Hz) relay in the art. Excessive flux density can occur due to either an overvoltage condition at normal frequency, normal voltage at a reduced frequency (underfrequency) or in general an excessive value of the ratio of voltage to frequency.
One important application of V/Hz overexcitation relays is to protect directly-connected generator unit step-up transformers. These unit transformers may be subjected to overexcitation during generator startup or shutdown, power system islanding, overloads and load rejection, any of which conditions can create an underfrequency or overvoltage condition and consequent overexcitation.
For example, DC field current is typically applied to a field winding of a generator when the machine is above 90% of its rated speed. If the field current is applied early (before sufficient generator speed is reached on startup), or not removed soon enough (after generator speed has fallen substantially during shutdown), the generator AC terminal voltage may be much higher than appropriate for excitation purposes relative to the actual electrical frequency, since frequency is proportional to generator speed.
Some generators are equipped with automatic voltage regulators which supply varying amounts of DC field current to maintain the generator AC voltage at a preset value at rated frequency. The preset value is reduced by the regulator if frequency falls substantially. An overexcitation relay advantageously is provided as backup protection for underfrequency relaying and Volts/Hertz control functions in the generator voltage regulator.
In another application, load tap changing (LTC) transformers and line voltage regulators may be subjected to excessive volts-per-Hertz during abnormal system frequency conditions due to their constant voltage control function. Also, the failure of an LTC controller may result in a runaway condition producing dangerously high voltage and consequent overexcitation. An overexcitation relay associated with an LTC transformer provides overexcitation protection for the transformer while allowing a wide range of voltage control operation.
In the prior art it has been known to produce an integral of the system voltage and compare it with a preset level to determine when excessive volts-per-Hertz is present. However, the process of integrating is time-consuming, and an overexcitation relay which operates more swiftly is desirable. Also, it has been known to provide a time trip function in which a condition of excessive volts-per-Hertz causes a timer to eventually trip a circuit breaker.
During overexcitation heat accumulates in the protected equipment. When and if the overexcitation ceases, the equipment cools. It has been known to reset a volts-per-Hertz relay in a predetermined period of time after an excessive volts-per-Hertz condition has ceased regardless of the degree of excessive volts-per-Hertz and the time during which that condition has persisted. It would be desirable to provide a volts-per-Hertz relay that actually and rapidly simulates the real heating and cooling characteristics of protected apparatus.